net: lan966x: Add spinlock for frame transmission from CPU.

The registers used to inject a frame to one of the ports is shared
between all the net devices. Therefore, there can be race conditions for
accessing the registers when two processes send frames at the same time
on different ports.

To fix this, add a spinlock around the function
'lan966x_port_ifh_xmit()'.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>